As roads deteriorate they may become disturbed and uneven. Settling of pavement and settling of layers underneath the pavement are examples of deterioration phenomena that cause roadway surfaces to become uneven. Potholes, thermal cracks, and alligator cracking may also cause significant roadway bumps and dips. Such deterioration may damage vehicle shocks, tires, and springs when driven upon. Smoother roads may reduce vehicle damage and accidents while affording drivers a safer and more comfortable roadway experience.
Damaged pavement can be repaired and even partially replaced. However, such do not always restore a pavement surface to the smoothness to that of the new originally installed pavement. To restore smoothness, irregularities have been reduced by using machinery that grinds the surface of the payment smoother, particularly on high-speed highways. The intent is to restore rideability by removing surface irregularities caused during construction or through water penetrating the asphalt pavement or water penetrating open cracks.
Uneven pavement surfaces can be smoothed by grinding. This can be relatively inexpensive when compared to other maintenance options. Notwithstanding, road grinding for road smoothing has been used in only a limited variety of applications. This is in part due to the fact that the grinding requirements vary. Not all pavements require the same amount of road grinding. Some roads vary significantly in altitude over short distances while other roads vary little in altitude over long distances. Some pavements are relatively smooth, while other pavements are comparatively uneven. Further, paved surfaces often have irregularities and bumps from a number of causes that can cause discomfort for riders and problems for vehicles driving over them. Some pavements may be rough in only certain localized regions, while other pavements may have roughness over their full surface.
Pavement smoothness is a factor relating to both safety and comfort. Smoothing a pavement surface is not reducing the surface to a flat horizontal surface. Rather it involves removing irregularities in the pavement profile while following the primary profile of the pavement.
One factor in defining smoothness is roughness. Roughness relates to large irregularities and is a general description of the forces imparted to a vehicle that cause sudden movements of the vehicle and deflection of the suspension. Another factor in pavement smoothness involves the surface texture of the pavement surface. While roughness primarily involves vehicle suspension deflection and dynamic tire loads, the surface texture is involved more with the interaction between the road surface and the tire footprint. Surface texture relates to generally periodic irregularities that have wavelengths shorter than those of roughness, between about 50 mm and 0.5 mm. Surface texture is partly a desired property and partly an undesired property. Short texture waves, about 5 mm, act as acoustical pores and reduce tire/road noise. On the other hand, long wave texture irregularities increase noise. Texture can provide wet road friction, especially at high speeds, but excessive texture increases rolling resistance and thus fuel consumption. The goal in smoothing is to provide an optimum surface texture. Texture on the surface of about 0.1 to 0.25 mm from peak to valley is desired for tire traction and for removal of air trapped under passing tires.
Various grinding machines have been used for various surface grinding applications. These essentially fall into two types, (1) rotomills and (2) diamond grinders. Rotomills generally comprise a drum rotating on an axis parallel to the pavement surface that is carried on wheels over the surface in a direction transverse to the rotating axis. Teeth with hardened tips are spaced on the drum to provide a cutting action as the drum rotates. Rotomills are primarily designed for rapidly removing large quantities of pavement. Attempts have been made to modify rotomills to provide some surface smoothing. However, to change a machine originally developed for grinding be removing large amounts to a machine for the precision removal of a thin amount material with the purpose not to remove the material but to leave a smooth pavement surface, has not been fully successful. This is the case even for so called micro grinders. Because of several limitations, rotomills have not been able to accurately perform road smoothing of less than about ¼ inch (6 mm). This is apparent when driving over a surface ground by a rotomill. It is not smooth to the driver or the vehicle.
One of the reasons rotomills are generally not suitable for grinding is that the base of the wheels supporting the drum (usually based upon standard wheeled truck highway transport systems) is too narrow. Rotomill cutting drums wide enough to smooth an entire travel lane are commonly wider than the wheel base. Accordingly, any difference in wheel height is amplified at the ends by the overhang, causing the cutting drum to be raised a little more on one side than another, which is unacceptable for precise road grinding tolerances.
In many smoothing machines there is some kind of leveling system so that the cutting height of the drum doesn't respond to every irregularity in the pavement. This can be, for example, a system that adjusts the height of the drum based upon vertical movement of a wheel traveling ahead of a drum. The leveling systems in rotomills commonly do not have a long wheel base from front to back necessary for this accurate road grinding. The too-short wheel base makes the height of the cutting wheel too reactive to short-term variations creating at best a wavy ground surface.
Leveling systems can also include automatic controls comprising sensors and hydraulic systems that can adjust cutting drum height. Another reason that rotomills are inaccurate for road grinding is because the automatic controls used in rotomills are inadequate to compensate to small changes in pavement height, particularly in the range within about ⅛ inch (3 mm). This inadequacy is caused by the automatic control's inability to hold the cutting drum steady. Although, automatic controls react to changes in the pavement surface and readjust cutting height accordingly, they also respond other extraneous movements, such as small movements in the wheeled trucks, cylinders, etc. The automatic controls interpret these small movements as road irregularities. Compensation to these ghost irregularities can result in a surface that is less smooth than the original. In addition compensation by the automatic controls can increase these extraneous movements, resulting in an increasing bouncing feed-back loop. When the rotomill is stationary, this bounce results in more cutting, leaving a scallop or divot. For, example, in just ten seconds while the grinder is stationary to replace a full dump truck, this can bounce the road grinding machine enough to leave a small divot in the pavement.
In addition, most rotomill automatic controls have an ability to be set in increments of 1/10 inch (or 2/20 inches (2 mm or 1 mm)). If, for example, a ⅛ inch (3 mm) cut is desired, the automatic controls only give one setting which may be too small or too great for the situation. In conclusion, use of automatic controls is not sufficiently accurate for highly accurate road grinding.
Another problem with rotomills is their sheer size and complexity. The precision required for smoothing leaves little room for operator error. Operators typically have training and experience in grinding and removal where for example, in milling 100 to 300 tons per hour cutting several inches deep. On the other hand, operators have limited training in removing material for smoothing, for example, 0.1 ton per hour cutting 0.05 inches deep. But, even with a trained operator the complexity of the machine and the highly variable pavement conditions make it difficult to reproducibly operate a machine at the same grinding height over time and from one job to the next. This makes conventional road grinding rotomills inaccurate for smoothing and bump removal. Basically, the concept of providing precise reproducible results is only theoretical under ideal conditions, and cannot be achieved in real world conditions.
Another problem derives from the carbide teeth commonly used on rotomills, which may wear down and create uneven patterns. In addition, tooth height is variable because, teeth used on rotomill grinding drums exhibit radial variation when installed and slippage during used of as much as ¼ inch (3 mm). Since many bumps in smoothing are cut only 1/20 inch (1 mm) and the average cuts are often 1/10 inch (2 mm), this radial variation is not acceptable for precision grinding.
Another drawback that makes rotomills inaccurate is the size of the tooth spacing. Tooth spacing on standard rotomills, even so called micro mills, is too wide to create a surface texture necessary for reducing tire noise. Larger teeth and wider spacing between the teeth may decrease precision compared to diamond grinding. The result of the spacing and variability of tooth height is a grinding drum that is variable over its surface, making it impossible to achieve precise and reproducible results. Consequently, rotomills tend to produce a rough and noisy riding surface.
Forward speeds of rotomills are not finely tuned at extremely slow speeds. Rotomills are designed for relatively fast working speeds of 40-150 feet per minute. Much slower speeds are required to provide the texture needed to leave the milled surface open to traffic. Because of this, rotomilled surfaces are almost always covered with new pavement within a few weeks.
Basically, rotomills are best used for removing large amounts of material and are used for demolition, mining, excavation, removing pavement, and the like. Their suitability for accurately and suitably smoothing pavement has not been demonstrated.
During a cutting operation, properties of a rotomill cutting drum relating to cutting vary significantly. In addition, the cutting properties from job to job vary and are not reproducible. Such variance of cutting in rotomills acceptable where there is gross removing of material and any smoothing is limited to mitigation of large irregularities. However, this variance is unacceptable for precision smoothing.
Diamond Grinders
Diamond grinders can grind bumps accurately, and can restore a pavement smoothness to resemble newly applied pavement. They can be used for rehabilitation of concrete pavement surfaces by grinding a smoother surface, and also to smooth new asphalt pavements beyond the smoothness afforded by the asphalt laydown machine.
Diamond grinding involves removing a thin layer at the surface of pavement using closely spaced diamond disc saw blades. The blade assembly is run at a predetermined level across the pavement surface, which produces saw cut grooves. The uncut concrete between each saw cut breaks off more or less at a constant level above the saw cut grooves, leaving a ground surface (at a macroscopic level) with longitudinal texture. The diamond disc saw blades are a composite of industrial diamond particles and metallurgical powder matrix. Use of the blade ablates diamond particles from the cutting surface to expose new diamond particles, which changes the height of the grinder, and compromises the precision of the grinding.
Major deficiencies of diamond grinders are slow speeds, and limited grinding depths. The cutting rate for diamond blades is inherently slow and cutting to depths of more than ⅛ inch (3 mm) is difficult without reducing forward speed, which is already slow. As the cutting depth is limited, a diamond ground area can still be too high. Bumps are sometimes ground, but remain a bump due to the limited depth of cut of diamond grinders.
In addition, pavement smoothing or grooving with diamond grinders using diamond saw blades leave vertical edges along the cut path. While vertical edges are a benefit to deep concrete cutting, they create a hazard to the traveling public. The vertical edge, if on the downhill side of the road, may increase standing water time which leads to raveling of the surface, and spalling at cracks and joints. Surface deterioration occurs first in areas of high instability, which are at the vertical edges.